Auto focus is a bit slow indoors (at night) taking pictures of a moving baby. But I'm picky like that.

Long time visitor, but created an account to post a review of this lens. For basis of comparison, I have the Canon L 24-70 2.8 II and the 17-40 4.0. I knew I needed IS for the 70-200, and was looking at Canon's 4.0, 2.8 II and this lens (didn't consider the Sigma version). After 2 weeks worth of use, I can definitely say I'm very happy with it and will be keeping it. I haven't found anything that justifies the extra $1k price for the Canon 2.8 IS II.

Build quality is great. Blindfolded, I couldn't tell this from a Canon L. This thing is solid, and I really like the all-black color compared to Canon's less discreet white lens. Haven't tested the weather sealing/proofing, but I live in the Pacific Northwest so I'm sure I'll have plenty of opportunities.

Image Quality is right up there alongside my L's. Colors are rich and vibrant. Super pleased with it.

The USM (ultra sonic motor) is amazingly quiet and you can barely hear anything.

Price is fantastic. With the savings, I picked up a Kenko 1.4x TC and am really pleased with a 98-280mm 4.0 equivalent lens.

Auto-focus is probably the only downside I've found. It can take a second or two to lock on but that's indoors, at night, taking pictures of my 6 month old baby who is starting to move now. Of course, I'm zoomed in at 130mm+ so it could be me and my shaky hands.